6 ZnS Cu2HgI4 Voids and conductionDisordered high-T (right) and ordered low-T (left) unit cell of Cu2HgI4. In the disordered phase, there are on average two Cu+ ions and one Hg2+ ion for every four Iˉ anions. However, the positions of these cations are not fixed – in any given unit cell, it is expected that these three cations will occupy a random subset of the four potential tetrahedral Cu/Hg sites.Cu2HgI4Single (left) and doubled (right) unit cell of ZnS (zinc blende structure)Voids and conduction

7 Ionic Conduction Two cations of the same size, different charge:which held less tightly by lattice?Smaller chargeTwo cations of same charge, different sizes:- which moves more easily through lattice?Smaller size